In the measurement of radioactivity it is now common to employ the "coincidence counting" technique, wherein events relating to radioactive decay are detected in two or more detectors within a given time interval in order to eliminate various sources of error which would be introduced if only one detector were used. One of the most widely used devices for the measurement of radiation from radioactive substances is the scintillation counter. The basic element of a scintillation counter is a scintillation medium which absorbs incident radiation and emits photons as a result. Many of the emitted photons are incident upon a photo-cathode in a nearby photomultiplier tube, and the output of the photomultiplier tube is a measurable electrical pulse having a magnitude which is approximately proportional to the energy of the incident radiation.
A significant problem encountered in such use of scintillation counters is that there are a number of extraneous phenomena unrelated to the radioactivity of the sample, which produce output pulses from the photomultiplier tube of a scintillation counter. Such phenomena have been characterized as "singles" events, derived from extraneous factors, such as background or tube noise, or the like, independent of any detected radiation. By using the well known "coincidence" technique, the detection of "singles" events is substantially eliminated. This involves the use of two photomultiplier tubes respectively disposed on different sides of the sample. The effect of true radioactivity in the sample can typically produce emission of many photons simultaneously, or nearly simultaneously. Thus, there is a high probability that such an event will be detected by both photomultiplier tubes at nearly the same time. A "singles" event, however, would generate an output pulse from only one of the phototubes. Coincidence detectors distinguish legitimate pulses produced by a sample or source being measured from those produced by background or noise factors, on the basis of the degree of coincidence of the outputs from the two photomultiplier tubes.
In the case of positron imaging systems, two-input coincidence detectors have been commercially available for many years, but so far as is known, no four-input coincidence detectors exist in the prior art. The functional requirements of a system with four groups of detectors could be met by using six conventional 2-input detectors, but only at considerably greater cost than when using a single 4-input detector as will be presently described herein. Use of a single 4-input detector has the further advantage that the effective coincidence interval is the same for all combinations of detectors.
A preliminary search of the prior art yielded the following prior U.S. patents of interest;
Somerville, U.S. Pat. No. 2,939,013, Rosenstock, U.S. Pat. No. 2,999,157, Jordan, U.S. Pat. No. 3,560,744, Paine et al, U.S. Pat. No. 3,609,353, White, U.S. Pat. No. 3,792,255, Frungel, U.S. Pat. No. 4,044,258, and Luitwieler, U.S. Pat. No. 4,049,966.